C# – ASP.NET MVC DropDownList Selected Value Problem

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I have read a ton about this problem, but I cannot figure it out.

I have a dropdownlist on my view that is used for paging. You choose the page you want to go to and the form gets submitted. When the view is returned, the requested page is shown. The dropdownlist changes to reflect the newly shown page. This is working fine.

My problem is this:
I have 2 submit buttons, one for next page and one for prev page.
In the controller, if one of the submits have been pressed, the page# should be incremented or decremented by 1. This is happening, and the correct page of data is being shown. But, the dropdown list will not, NO MATTER WHAT I DO, reflect the new page number when the view is shown.

Basically, if the dropdown is changed by the user, it will reflect those changes when it is sent back after a form submission. If I change the page # programmatically, the dropdown will not change, even though the correct page of data gets shown.

Here is the action method:

public ActionResult Results(TechSearch search, string NextPage, string PrevPage)
{
    if (NextPage != null) search.Page++;
    if (PrevPage != null) search.Page--;

    int resultCount = search.GetResultCount(Desk);

    List<int> pages = new List<int>();
    int pageCount = (int)Math.Ceiling((decimal)resultCount / search.PageSize);
    for (int i = 1; i <= pageCount; i++) pages.Add(i);        

    ViewData["pages"] = new SelectList(pages, search.Page);            
    ViewData["pageCount"] = pageCount;
    
    return View(search);
}

And here is the relevant part of the view:

<input type="submit" value="<" name="PrevPage" />
Page
<%=Html.DropDownList("Page",(SelectList)ViewData["pages"]) %>
of
<%=ViewData["pageCount"]%>
<input type="submit" value=">" name="NextPage"  />

Please help.

EDIT

Page 333 of Pro ASP.NET MVC Framework by Steven Sanderson:

How Input Controls Get Their Values

Each of [the HTML Helpers for Rendering Input Controls] tries to
populate itself by looking for a value
in the following places, in this order
of priority:

  1. ViewData.ModelState["controlName"].Value.RawValue
  2. value parameter passed to HTML helper method, or if you’ve called an
    overload that doesn’t include a value
    parameter, then
    ViewData.Eval("controlName")

ModelState is a temporary storage area
that ASP.NET MVC uses to retain
incoming attempted values plus binding
and validation errors. Notice that
it’s at the top of the priority list,
so its values override anything you
might set explicitly. This convention
means that you can pass an explicit
value parameter to act as the helper’s
default or initial value; but when
rerendering the view after a
validation failure, the helper will
retain any user-entered value in
preference to that default.

I have verified that ViewData.ModelState["Page"].Value.RawValue does contain the unexpected page number that is pestering me. What's the best way to work around this?

Best Answer

The problem was that ModelState was holding the dropdownlist's passed in value. For the HtmlHelper input controls, ModelState is given priority over explicitly set values when displaying a value in the input control.

Here is what I have done in my action method to fix the problem:

ModelState.Remove("Page");
return View(search);

I am interested in hearing other ways of fixing my problem.

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